What is a primary effect of grain boundaries on diffusion?

• A. Boundaries slow diffusion everywhere.
• B. Boundaries do not affect diffusion.
• C. Boundaries can serve as fast diffusion paths for certain species.
• D. Boundaries have no influence on diffusion.

Answer: (C)

Grain boundaries provide faster diffusion pathways for certain species because their structure is more disordered and has higher free volume than the perfect crystal lattice. This lowers the energy barrier for atomic jumps along the boundary, so atoms can migrate more readily there than through the orderly lattice inside grains. In a polycrystal, diffusion can occur both through the lattice and along grain boundaries, and for many small atoms or impurities, the boundary route dominates at practical temperatures. So the primary effect is that grain boundaries can act as fast diffusion paths for some species, rather than slowing diffusion everywhere or having no influence.